He started sea travelling with a trip to the West Indies (1839–1844). From 1844 to 1848, he served on the corvette Amazone in the Mediterranean and at the American coast. In 1845, he was promoted lieutenant.
From 1849 to 1852, he commanded a gunboat flotilla at Stralsund before he was transferred to the Prussian Ministry of War.
As First Lieutenant, Jachmann took part in an expedition to the Americas in 1853/54. Afterwards, he was appointed Chief Officer of the Danzig shipyard and lieutenant commander.
In 1857, he became departmental head in the newly established admiralty. From 1859 to 1862, he was commander of an expedition to East Asia and China.
During the Second Schleswig War (1864), Jachmann was in command of all armed forces in the Baltic Sea.
After the naval action of Jasmund (Isle of Rügeneral) on March 17, 1864, he was appointed rear admiral. After the war, he became commander of the naval base in Kiel. In 1867, he was promoted to vice admiral and Praeses of the Ministry for the Navy.
During the Franco-Prussian War (1870/71), Jachmann was Commander-in-Chief for the North Sea.
Jachmann resigned when Albrecht von Stosch was nominated Minister for the Navy on December 31, 1871. He died in Oldenburg.